1. Field of the Invention
This invention relates in general to a method for driving a display device. More specifically, the present invention relates to a method for driving a passive matrix organic light emitting diode (PMOLED).
2. Description of Related Art
FIG. 1 schematically shows a typical PMOLED array for describing its conventional driving method. Referring to FIG. 1, a PMOLED array 100 comprises a plurality of segments X1, X2, X3, . . . , Xn and a plurality of scan lines Y1, Y2, Y3, . . . , Ym, wherein the segments X1, X2, X3, . . . , Xn and the scan lines Y1, Y2, Y3, . . . , Ym, are respectively intersected to form an array. The PMOLED array 100 further comprises a plurality of OLEDs 12, and each OLED 12 is connected between one segment and one scan line to form the PMOLED array 100. In addition, each of the segments X1, X2, X3, . . . , Xn are connected to a constant current source I, and the constant current source I provides a PWM (pulse with modulation) constant current to drive the OLEDs 12 connected to the segments Xi (i=1˜n).
FIG. 2 is a schematic timing diagram for driving segments of the PMOLED. According to the conventional method of driving the PMOLED, when one scan line is selected by asserting a scan pulse during a particular horizontal line period T, the segment is first pre-charged. In this way, parasitic capacitors of the selected and the non-selected pixels on the segment are pre-charged with a constant voltage, so that the pixel is in a critical state and prepared to be turned on. After the pre-charge period T0, the PWM constant current I is applied to the selected segment to drive the OLED 12 of the pixel.
In the cycle of applying the PWM constant current, the gray scale of the selected pixel can be presented. Ideally, the gray scale of the selected pixel is proportional to the pulse width of the PWM constant current. However, it is very difficult to choose a proper voltage to pre-charge the segment. If the pre-charge voltage is not properly chosen, there might be an under pre-charge or an over pre-charge problem. As a result, the pixel might be over pre-charged and thus a poor display contrast occurs. Alternatively, the pixel might be under pre-charged, and missing gray scales occur at low gray levels.
FIG. 3 is a concept illustration showing an over pre-charge and an under pre-charge according to the conventional PMOLED driving method. In FIG. 3, the brightness and the gray scale of the pixel are normalized to 1. Referring to FIG. 3, the diagonal solid line represents the ideal condition, i.e., the brightness and the gray scale are proportional. For example, the brightness “0” is corresponding to the gray scale “0”. However, if the under pre-charge condition occurs as represented by the line II, missing gray scales x occurs. Namely, when the brightness is “0”, its corresponding gray scale is “x”, rather than “0”. The gray scales smaller than “x” are disappeared, and the gray scales 0 to x−1 can not be displayed for the selected pixel. On the other hand, if the over pre-charge condition occurs as represented by the line I, the contrast of the selected pixel becomes poor because the pixel has been light ON even at gray scale 0.
As described above, since it is difficult to select a suitable pre-charge voltage to pre-charge the selected segment, the over and the under pre-charge conditions always occur, and the image quality for the PMOLED display device is adversely affected. Therefore, it is desired to a new method to overcome the over and the under pre-charge issues due to the pre-charge process without changing the basic circuit design.